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After Mary Thompson moved to Greensburg, Pennsylvania, in the mid-1990s, her sick mother told her something she never forgot.

"She told me to be careful in Greensburg because I had a sister that was murdered up there," Thompson told The Washington Post.

Thompson's mother died in the early 2000s, but her words stayed with her daughter - because she did have an older sister who disappeared on a fall day in 1967, when Thompson was only 4, and was never found.

From what Thompson was told, 13-year-old Teala Patricia Thompson left her home in the Homewood neighborhood in Pittsburgh in the early morning on a September day in 1967. That was the last time her family saw her. They reported her disappearance to Pittsburgh police, Thompson said, but investigators had come up empty.

Teala was identified this past week. Her body had been buried in an unmarked grave in a cemetery potter's field next to the Westmoreland County Prison in Greensburg.

The discovery came after nearly two years of a cold-case investigation led by Pennsylvania State trooper Brian Gross, who was assigned the case in October 2014. One of his crime unit supervisors told him to look into an open homicide investigation that involved an unidentified girl whose body was discovered in a landfill in Salem Township, Pennsylvania, in 1967 - the same year Teala disappeared. The court later ordered the body to be buried in that potter's field, Gross told The Washington Post.

Last October, investigators received a court order allowing them to exhume the body. A dental charting done in the 1960s showed that the remains definitely belong to the girl from the landfill, Gross said. At that point, it was just a matter of finding out who she was.

Thompson heard about the exhumation in the news and immediately called Gross. "It's my sister," Thompson told Gross, remembering her mother's words.

Investigators obtained DNA samples from Thompson and other relatives and sent them to the University of North Texas, which does DNA testing for the National Center for Missing and Exploited Children.

The 52-year-old mother of four said she has waited all her life for that news, which gave her the best feeling of her life, and the worst feeling of her life. Although she does not remember much about her sister Teala - her mother had 10 kids - she grew up wondering where she was, and that question haunted her family for decades.

"We're grateful, but we're still hurting," she said. "We're happy, but we're hurting."

Teala would've been 62 today. Thompson said her family would like to give her a proper burial. She plans to give the remains to her older sister Jerry Denson of Saylorsburg. Denson and Teala were about the same age when she disappeared. They played together, Thompson said. They were best friends.

In 2015, 460,499 incidents of missing children were reported to the FBI's National Crime Information Center, according to the FBI. That number was 466,949 in 2014.

A 2012 nationwide survey by the nonprofit RAND Corporation found that police agencies cleared about one in five cold-case investigations. Homicide is the most common type investigated by agencies, followed by sexual assault and burglary. The survey also showed that funding for cold-case investigations is often thin. Only 7 per cent of responding agencies have dedicated cold-case units, and 14 per cent had formal procedures in determining which cases to investigate.

The Pennsylvania State Police, where Gross has worked for 25 years, is one of those law enforcement agencies that have cold-case units. Gross said he will retire next week, and another investigator will take over Teala's case. Although the discovery of her body gave her family some resolution, it isn't closure, Gross said, because the girl's killer has yet to be found.

Thompson and her family have always had their suspicions. Teala used to help with work at a now-closed dry cleaner in the Homewood neighborhood. They believe a man who worked there is responsible, but no one has ever been charged.

"We just want to know what happened to her," Thompson said. "How did she die?"